Heat-sensitive copying paper



c. s. MILLER ET AL 2,663,655

HEAT-SENSITIVE COPYING PAPER Dec. 22, 1953 Filed May 15, 1952 2 Sheets-Sheet l Dec. 22, 1953 c. s. MILLER ET AL 2,663,655

HEAT-SENSITIVE COPYING PAPER Filed May 15, 1952 2 Sheets-Sheet 2 if? f zwfparen/ 50p/00N? Patented Ecc, Q22, M53

HEAT-SENSITIVE COPYING PAPER Carl S. Miller and Bryce L. Clark, St. Fael, Minn.,

assignors to Minnesota Mining & Manufacturing Company, St. Paul, Minn., a corporation of Delaware Application May 15, 1952, Serial No. 288,664

(Cl. 11i-36) 7 Claims.

This invention is concerned with duplicator sheet mate ial or copying-paper useful in preu paring copies of printed matter or the like, and with compositions and methods related thereto. The invention particularly contemplates heatsensitive coatings and coated sheet materials of a novel type, and methods of utilizing such materials in preparing duplicate copies of printed or other graphic subject-matter oy simple exposure to high intensity irradiation and without subsequent developing or fixing operations.

This application is a continuation-in-part of our copending application Serial No. 747,339, led May 1G, i947.

Prior art copying-papers of which we are aware, which are subjected to exposure to vlsiole light or similar forms of radiant energy in the preparation of copies of graphic subject-matter, must 'ce protected from light before use, and require various processing steps, in addition to the initial controlled exposure, for the production of visible light-stable copies.

Our new heat-sensitive copying-paper, on the contrary, is not itself affected by radiant energy such as visible or ultra-violet light, and may therefore be left exposed to such radiations both before and after printing. Nevertheless, radiant energy may he employed as the sole agency for the production of visible light-stable and permanent copies of graphic subject-matter on our novel copying-paper, or on other surfaces carryour novel heat-sensitive compositions, by processes as hereinafter set forth.

rThe processes by which We are enabled to achieve this result, e. g. in the preparation of copies of the printed pages of books or the like, involve the irradiation of the printed surface, the resultant formation of an elevated-temperature pattern corresponding to the graphic subject-matter of such printed surface, and the utilization of this elevated-temperature pattern in the development of a facsimile copy oi the graphic subject-matter on the heat-sensitive copying-paper.

Formulas may be found in the prior art for various heat-sensitive paints, coated sheet materials, and the like. All such products of which we are aware exhibit one or more significant deficiencies when compared with our novel heatsensitive composition and coated products. They may be sensitive to light as well as heat. They may require an extremely high temperature, or a relatively long time at a specific temperature, or heating over a wide range or" temperature, to produce a visible change of sucient intensity. The change obtained at elevated temperatures may be reversible on cooling, i. e. the product eX- hibits thermotropy. These and other deficiencies of prior art materials are avoided by the use of our novel compositions and products, particularly Cil in connection with the copying of printed matter such as books, drawings or diagrams, pictures, etc. by inea-ns of our novel copying-paper herein i. and illustrated e heat-sensitive compositions ci our invenion are comprised of solid ionizabie reactants which are potentially chemically Capable of ir- 'eversioly and rapidly e'cting normal rooin eniperature in a direct ion-exchange electron acceptor-donor chemical reaction to produce visi ly different reaction product, but i. oh are normally maintained in physically disnnct relationship hysioaliy prevents -rom so reacting. The structure is so designed that an increase in temperature to a predetermined level allows the reaction to take place. The reaction s believed to be initiated by the inciting, or softening, or other physical change, of one or more of the reactive substances. Tlvhatever the s cii'ic physical change employed, the applica of heat results in an immediate reaction oi the reactants and formation of a colored, opaque, or otherwise visibly diierent reaction product.

The ramo of reaction obtainable by this means particularly advantageous where the reactive material in sheet iorin is to be "i heat-sensitive copying-paper. For the sost eifective reproduction of drawings or he like containing fine lines as well as inassive darla areas, a high ccnrast number, as hereinafter darned, must be obtained when 'the sheet is heated through a narrow range of temperature not greater than about 2 C. and preferably about 5o C., within a period of time not greater than one second, and preferably within about onetenth of a second, in order to obtain desirable contrast and detail.

Where these limitations are exceed f productions obtained may be blu-rre' or otherwise of poor quality. T nple, heat-sensitive sheet material which ns over a wide range or" temperature, e. g. which provides a contrast number of 0.2 when heated to To? C. but does not reach a contrast number oi until heated to 110 C., will ordinarily not produce a print having good contrast between light and darli areas.

A convenient method of deterinin' reaction as well the required activation for a particular copying placing portions of the sheet metal surfaces each at a Our preferred copying-papers, pshow a reaction troni the original s to a c intensity or opacity equivalent to a contrast n" bei' of at least about and preferably about when heated at the requir :l temperature level and within a temperature range of incre than 3 about C., and within a contact time of not more than about one-tenth second.

In determining the contract number of a copying-paper, suitably exposed samples having printed (darkened) and unprinted (undarkened) areas are held against a fiat magnesiumL carbonate block (a standard magnesium carbonate surface as deiined in the paint industry). A beam of white light (north exposure daylight) is directed against the outer surface of the sample at an angle of 45, and the intensity or the light reflected normal to the surface is measured by means of a suitable photometer. The contrast number is defined as the following ratio 11F-Id I .i where I represents intensity, and subscripts u and d refer to measurements made over the undarkened area and darkened area, respectively.

The following examples of specific compositions and structures will serve more clearly to point out and explain the novel concept here involved, but are not to be construed as limitative.

Example 1 One pound of nickel acetate (tetrahydrate) was dispersed in one liter of anhydrous butyl acetate by grinding in a ball mill. Similarly, one

pound of calcium sulfide was ground into one u Volumes Nickel acetate dispersion 20 Calcium sulfide dispersion 5 Binder This mixture was applied in a thin layer to I various supporting surfaces, e. g. cellophane or cellulose acetate film, paper, wood, leather, etc., and dried at room temperature. The dried iilm was faintly greenish-white in appearance. When heated, the iilm rapidly became intensely black and opaque.

The reactive ingredients of Example 1 are found to be inter-reactive under proper conditions at normal room temperature, with the production of a dark-colored and opaque reaction product. For example, the addition oi a small amount of Water to a mixture of nickel acetate and calcium sulde in powder form initiates the reaction between the two compounds. 'it is believed that the reaction obtained in the coated sheet material is due to the increased vapor pressure, at an elevated temperature, of the water present in the hydrated salt, and/or the melting of the hydrous salt to a fluid state, resulting in a reactive mingling of the nickel compound and the sulfide.

Example 2 Strontium sulfide and nickel acetate tetrahydrate were ground up in a 21% solution of 5 second nitrocellulose in butyl acetate, in the ratio of one gram each of insoluble strontium sulde and nickel acetate tetrahydrate particles to three milliliters of nitrocellulose solution. The mixture was coated on cellophane and dried at room tem perature. The sheet was light gray in color, changing to an intense black when briedy heated to 115 C. It could be used as a heat-sensitive copying-sheet by methods described herein, producing copies of good contrast and detail.

Barium sulde and nickelacetate tetrahydrate produce similar results. In a specific formulation, 20 ml. of a dispersion of one pound of the nickel acetate in 1000 ml. of butyl acetate, 50 inl. of a 28% nitrocellulose solution in butyl acetate, and 10 ml. of a dispersion of one pound or barium sulde in 1000 ml. of butyl acetate were mixed and coated on paper, and dried at room temper-- ature. A protective surface sizing of a 15% soluu tion of nitrocellulose in butyl acetate was applied over the heat-sensitive coating and dried. The linished copying-sheet was light yellowish green in color, and produced dark-colored brown-black copies when used as a heat-sensitive copying paper by methods above indicated. Tested on a metal test block, the Visible reaction was found to occur at approximately S33-105 C.

Other film-forming cellulose derivatives, as well as various non-cellulosic binders such polyvinyl butyral, rubber, polyvinyl acetate, rul s ber hydrochloride and the like, may be substin tuted for the specinc binder above mentioned, provided the volatile solvents required for such binders are at the same time non-solvents for, and non-reactive with the particulate solid comn ponents.

Example 3 Another combination of heavy metal salt and alkaline earth metal sulfide which has produced useful copies when employed in structures such as that ci Examples l and 2 contains solid particles of nickel stearate and barium sulfide in a nitrocellulose binder. l"She nickel stearate, prepared by precipitation from a mixture or aqueous solutions of nickel acetate and the sodium salt of commercial stearic acid, is a waxy solid at temperatures below 60 C., and liquid at 120 C. fne coated and dried sheet was dipped into a Llzl blend of naphtha and mineral oil and again dried; the residual mineral oil improved the heatconductivity properties of the sheet. On a metal test block, the sheet changed rapidly from light yellow to dark brown at about 113 C., and could be used in making eiective facsimile copies of typewritten letters and the like by the methods herein described.

Example 4 The reaction of Example 3 can be obtained at a lower temperature by incorporating in the heat-sensitive nlm a small proportion of a free organic acid. The following combinations oi reactants, suspended in particulate form in a 5% solution of crepe rubber in heptane, coated on flax paper, and dried at room temperatures, were each effective as heat-sensitive copying-papers and produced the indicated color change at the temperature specified. n

1. Nickel palmitate-barium sulde-stearic acid; changed from light green to intense black at C. Calcium sulde produced a light gray sheet with otherwise identical properties.

2. Nickel palmitate-bariurn sulfide-palmitic acid; ghanged from light gray to intense black at 3. Nickel caprylate-calcium sulfide-stearic acid;- changed from gray to intense black at 50 C. Crotonic acid in place of stearic acid raised the conversion temperature to '72 C.

Many other compounds in addition to the several materials previously enumerated have additionally been found to produce adequate color differences when combined as closely spaced particles in a film-forming binder as described in the above examples. Cobalt and lead salts are,

for example, equally as eilective as the salts of nichel. Salts of silver, mercury, copper and iron are useful but less desirable, silver and mercury salts showing a tendency to darken on exposure ci the paper to sunlight and copper and iron salts being initially undesirably darli in color and hence providing less contrast in the resulting copy. Colcrless or lightly colored suldes such as zinc cadmium sulde, and magnesium sulde are eilective in formulations such as those of Example fi and for such purposes may be considered fully equivalent to the alkaline earth metal sulides specified in that example.

The mixture of reactive solid components is conveniently applied to paper or other supporting structure as a dispersion in a solution of a bonding agent in a suitable volatile vehicle, as disclosed in the above examples. The bonding agent assists in retaining the reactants on the surface of the support. However, other methods of applying the reactants to the supper ing surface and oi maintaining them in proper relationship thereon may alternatively be employed. For example, a polymerizable monomer may be substituted for the solution of bonding agent; after application, the monomer may be polymerized in situ to form a binder nlm. The powdered reactants may be dispersed within, or on the surface of, a ibrous web or other supporting structure in the substantial absence of any added bonding agent. Additionally, the use of a nlneiorming bonding agent, such for example as the nitrocellulose of Example l, as a self-supporting film as Well as a binder and carrier for the reactive ingredients is also contemplated. In this type of product, the film-forming composition containing the color-producing reactants may be coated and dried on a temporary support, and the dry subsequently removed by stripping to provide an exceedingly thin copying-paper.

Certain advantageous results may be obtained by proper selection and proportioning of the bonding agent as well as the reactants, and in our preferred compositions We therefore contemplate the use of a suitable inert bonding agent or combination of bonding agents in significant proportions. 'lhe degree of contrast obtainable With our copying-paper is readily controlled, for example, by suitably proportioning the relative amounts of binder and or reactants. Thus, increased contrast has been obtained by increasing the percentage of binder, and thereby presumably increasing the distance and the amount oi non-reactive material between or around the individual particles of reactants. Conversely, a reduction in the percentage of bonding agent re duces the contrast and increases the detail obtainable the resultant copy.

Changes in the particle size and shape of any one or all of the reactant materials, and in the relative amounts of the individual reactants, will also have some effect on the results obtained.

Stoichiometric relationships between reactants` to the sensitizing compositions of our invention. Additional surface coatings, e. g. of nlm-forming materials, may be applied as protective layers, or to impart desirable color, or for other purposes. Other modifications will be apparent.

In order to assure a clear understanding of the structure of our novel product and the manner in which it may be utilized, e. g. in the copying of printed matter, reference is made to the accompanying drawing, in which:

Figure l is a conventionalized and enlarged cross-section of one example of a suitable heatsensitive coating on a supporting member;

Figures 2, 3 and 4 are perspective views (partially cut away) of three different arrangements of a printed surface in relation to the coated surface of the copying-paper in obtaining facsimile reproductions of printed or other graphic matter on the heat-sensitive coated copying- .papers of this invention; and

Figure 5 is a diagrammatic, sectional view or" one example of structure, including a suitable source of radiant energy, may be employed for copying printed sheet material.

In Figure l, a heat-sensitive layer il?, consisting of tvo iinely-divided reactant materials Il and I2, the in idual solid particles of which are contained within a binder material I3, is supported on a supporting member I4.

n Figure 2, a transparent heat-sensitive copying-paper l5, consisting or" a heat-sensitive layer il) on a thin transparent backing it (both shown in cut-away section) is placed against a printed page l?, with the uncoated surface of the transparent backing l@ in contact with the printed characters i8 of which a facsimile copy is desired. The transparent backing it of Figure 2 is a specic member ol the class more generically represented Figure l by supporting member ld. 0n exposure of the composite to intense illumiA nation, as herein elsewhere explained, and in this case with the transparent heatesensitive sheet i5 being between the source of light and the printed surface, a differential darkening, or other visible change, indicated by darkened area i9, is rapidly obtained in the heatesensitive layer i corresponding to the printed character I8 therebelow.

The inished print obtained by the procedure outlined above in connection with Figure 2 is directly readable from the surface carrying the heat-sensitive layer. Transparency of the sheet material i5 to visible light is therefore not essential or subsequent reading of the copy obtained. Consequently the requirement of transparency here refers to the ability of the sheet to pass radiant energy of the wavelength employed in the copying process. The use oi visible light thus imposes the requirement or" visible transparency, as manifested, for example, in the construction embodying cellophane and described under Example l. With other wavelengths, e. g. in the high-energy infra-red region, sheet material which is transparent to the infrared but which may appear translucent or even opaque to the eye may be used and. is here contemplated.

ln Figure 3, the heat-sensitive layer of the (visibly) transparent heatesensitive copyingpaper l5 is placed directly in Contact with the printed characters l of the printed page il. High intensity radiation directed against the transparent backing iS results in a visible change, in the sensitive layer la, corresponding to the printedcharacters i8, indicated by the darkcned area le, (partially concealed in the drawing). The resulting copy is a direct facsimile of the printed original when viewed through the visibly transparent backing i5,

In Figure Ll, the heat-sensitive coating it is disposed, as shown in cut-away section, on a supporting member iii, which may be either transparent, translucent or opaque, but which should be at least reasonably non-conductive to heat. A thin opaque printed page 2t, which may or may not carry printed characters on the under side, but which has printed characters i8, of which a copy is desired, on the outer side, is placed against the sensitive layer le. Proper irradiation of the outer printed side of the printed results in the formation of Aa visible facsimile copy or darkened area is, on the sensitive layer ifi, corresponding to the printed characters i2, with no undesirable blurring or ghost-image formation being obtained from any similar printed characters which may be present on the under side of the page 2t.

In Figure 5, which illustrates in cross-section one means by which reproductions of printed matter may conveniently be made according to the principles set forth in connection with Figure 2, an electrically-activated high-intensity source of useful radiation such as light rays 23, cornpricing an incandescent line filament 2l in transparent evacuated envelope 22, is shown supported within a movable trough-hire reflector 2t of elliptical cross-section. Filament 'li is located at one focal line of the elliptical reflector; consequently light rays 23, emanating therefrom, are focused at the other focal line, which caused to coincide with the printed surface of printed page i'i, car ig printed or other graphic characters it of *auch a facsimile copy is desired.

Page il rests on the uncoated surface of a sheet of transparent heat-sensitive copyingpaper is, consisting of a transparent backing it and c, heat-sensitive coating le. The copyingpaper rests on transparent heat-insulating sheet material which may be an open-mesh screen such as sili: bolting-cloth, and this screen in turn rests on a transparent support 2t, Which may be a glass plate, preferably of heat-resistant glass. (Where support 25 is inherently sunlciently heat-non-conductive or is suitably roughcned or otherwise treated at the surface so as to present a non-conductive surface to the copying-paper it, the separate thickness of insulation E5 may be eliminated.) The copy-paper i5 and page i'i are held in close and heat-conductive contact by means of a heat-insulating cover 2i, which may be a wooden cover, as shown in Figure E., or may be constructed of any other suitable material including a soft and flexible material such as a pneumatic or hydraulic pressure-bag. If desired, the cover 'l, and the underlying printed page il', may be maintained at any desired temperature below the printing temperature of the copying-paper l5, by suitable ternperature control means. The elliptical reflector 2d, with its enclosed light source,` is movable across the surface of the thermographio assembly, as indicated by the double arrow 28, by suitable means (not shown), in a plane such that the light rays 23 are continuously focused at the printed surface of the printed page l'l. The rate of movement of the reflector assembly is adjustable, to compensate for differences in the thickness and heat conductivity of the page to be copied, here shown as printed page Il, and .also to compensate for differences in the ,Sensitivity of the temperature-sensitive layer it, the intensity of light output, and various other factors.

For rapid copying of text-book pages, letters, or the like with the apparatus of Figure 5, it has been found convenient to use a 200G-Watt coiled tungsten line-lament incandescent lamp, l0 inches long, With Pyrex glass envelope, mounted in an elliptical reflector 26. The resulting concentration of radiation is of such high intensity as to cause charring or burning if allowed to remain focused on the same section of paper for more than about one second. With this light source, and with temperature-sensitive copying paper such as that described under the various examples, clear unfogged facsimiles may be obtained. However, many other sources of radiation, including ordinary high-intensity incandescent lamps, photoash lamps, electric arc, infrared lamps, concentrated sunlight, etc., have been found to be useful but commercially less effective in various therznographic assemblies using our heat-sensitive copying-papers.

Many modications, both in preparing the reactive compositions and the coated materials, and in utilizing the products in the direct reproduction of graphic subject-matter, have been described. While the importance of each of these and other modifications in the commercial application of the principles involved will become obvious in view of the foregoing description, it will be even more apparent that the underlying principle, on which the novel phenomena herein presented are based, remains the same in all of the applications and modifications thereof. This principle involves the selection and 'blending oi two or more solid reactants in such a manner that the reactants are prevented from combining or reacting with each other until the blend is heated to or above a predetermined range or" temperature. When such a temperature is reached, however, the reactants are enabled rapidly to combine or react, whereupon there is produced a visible indication of such action, e. g. a color change. While the action thus obtained is of value under 2, Wide variety of conditions, it lhas been shown to be particularly valuable in making available, as a new article of manufacture, a heat-sensitive copying-paper suitable for the preparation of visible light-stable copies of graphic subject-matter by methods involving only the exposure of such matter to intense irradiation while in heat-conductive relationship with the heat-sensitive material.

Having now described our invention in terms of specific examples and embodiments, but Without intent to be limited thereto, what We claim is as follows:

1. A heat-sensitive copying-sheet for direct, high contrast, clear detail copies of graphic subject-matter as herein described, said copying-sheet comprising paper coated with a heat-sensitive layer containing, uniformly dispersed in -a film-forming binder non-fusing at temperatures below about iZG" C., a multitude of closely spaced solid particles of stable ionizable dissociable materials comprising a salt of heavy metal the sulfide of which is colored, and an allialine earth metal suliide, at least one of said heavy metal sait and said alkaline earth metal sulfide melting at a temperature within the range of about 6i?- 20 C. and being present in an amount such that on fusion and reaction With other materials a visible change is produced in the heat-sensitive layer, said materials being maintained in physically distinct and chemically inter-reactive relationship in said layer at normal room and storage temperatures, said materials capable of rapidly visibly reacting at normal room temperature in a direct ion-exchange electron acceptor-donor chemical reaction in a mutual solvent capable of permitting ionization of said materials and with the formation oi a stable, visibly distinct, highly polarized compound which is less dissociable than either of said materials, and said materials in said heatsensitive layer being stable at temperatures less than about 60 C. and being rapidly visibly interreacted when the copying-sheet is heated to 120 C.

2. A heat-sensitive copying-sheet in accordance with claim 1 and having over the heatsensitive layer a thin transparent protective surface sizing layer.

3. A heat-sensitive copying-sheet for making direct, high contrast, clear detail copies of graphic subject-matter as herein described, said copying-sheet comprising a support having low thermal conductivity, and a heat-sensitive layer containing, uniformly dispersed in a film-forming binder non-fusing at temperatures below about 120 C., a multitude of closely spaced solid particles of stable ionizable dissociable materials comprising a salt of a heavy metal the sulfide of which is colored, and an alkaline earth metal sulde, at least one of said materials melting at a temperature within the range of about 60-120" C. and being present in an amount such that on fusion and reaction with other materials a visible change is produced in the heat-sensitive layer, said materials being maintained in physically distinct and chemically inter-reactive relationship in said layer at normal room and storage temperatures, said materials being capable of rapidly visibly reacting at normal room temperature in a direct ion-exchange electron acceptordonor chemical reaction in a mutual solvent capable of permitting ionization of said materials and with the formation of a stable, visibly distinct, highly polarized compound which is less dissociable than either of said materials, and said materials in said heat-sensitive layer being stable at temperatures less than about 60 C. and being rapidly visibly inter-reacted when the copyingsheet is heated to 120 C.

4. Aheat-sensitive copying-sheet in accordance with claim 3 in which the heavy metal salt is a hydrated salt melting within the range of about fio-120 C.

5. A heat-sensitive copying-sheet in accordance with claim d in which the hydrated heavy metal salt is nickel acetate tetrahydrate.

6. A heat-sensitive copying-sheet for making direct, high contrast, clear detail copies of graphic subject-matter as herein described, said copying-sheet comprising a support having low thermal conductivity, and a heat-sensitive layer containing, uniformly dispersed in a ulm-forming binder non-iusing at temperatures below about 120 C., a multitude of closely spaced solid particles of stable ionizable dissociable materials comprising 1) a salt of a heavy metal the sulde of which is colored, (2) an alkaline earth metal sulfide, and (3) an organic acid melting at a temperature within thJ range of about -120 C., said materials being present in an amount such that on heating and inter-reaction a visible change is produced in the heat-sensitive layer, Said materials being maintained in physically distinct and chemically inter-reactive relationship in said layer at normal room and storage temperatures, said materials being capable of rapidly visibly reacting at normal room temperature in a direct ion-exchange electron acceptordonor chemical reaction in a mutual solvent capable of permitting ionization of said materials and with the formation of a stable, visibly distinct, highly polarized compound which is less dissociable than either of said materials, and said materials in said heat-sensitive layer being stable at temperatures less than about 60 C. and being rapidly visibly inter-reacted when the copyingsheet is heated to C.

7. A heat-sensitive copying-sheet in accordance with claim 6 in which the heavy metal salt is a nickel salt of an organic acid.

CARL S. MILLER. BRYCE L. CLARK.

References Cited in the le of this patent UNITED STATES PATENTS me Date 

1. A HEAT-SENSITIVE COPYING-SHEET FOR MAKING DIRECT, HIGH CONTRAST, CLEAR DETAIL COPIES OF GRAPHIC SUBJECT-MATTER AS HEREIN DESCRIBED, SAID COPYING-SHEET COMPRISING PAPER COATED WITH HEAT-SENSITIVE LAYER CONTAINING, UNIFORMLY DISPERSED IN A FILM-FORMING BINDER NON-FUSING AT TEMPERATURES BELOW ABOUT 120* C., A MULTITUDE OF CLOSELY SPACED SOLID PARTICLES OF STABLE IONIZABLE DISSOCIABLE MATERIALS COMPRISING A SALT OF A HEAVY METAL THE SULFIDE OF WHICH IS COLORED, AND AN ALKALINE EARTH METAL SULFIDE, AT LEAST ONE OF SAID HEAVY METAL SALT AND SAID ALKALINE EARTH METAL SULFIDE MELTING AT A TEMPERATURE WITHIN THE RANGE OF ABOUT 60-120 C. AND BEING PRESENT IN AN AMOUNT SUCH THAT ON FUSION AND REACTION WITH OTHER MATERIALS A VISIBLE CHANGE IS PRODUCE IN THE HEAT-SENSITIVE LAYER, SAID MATERIALS BEING 